Until now, silicone rubber has been used as electric power components such as a cable accessory for electric wire coating at a connecting portion of power cables because of its excellent heat resistance, electric properties, weatherability, and the like.
In particular, a silicone rubber having a high electric insulation is used as a coating material of a High Voltage Insulator (HVI) used at high voltages, and in branching and connecting the high voltage cable, there is frequently applied the cold-shrink connection method which is conducted by inserting a conductor to be branched and connected, into a silicone rubber insulator expanded in diameter in advance, and then by pulling out the separator to thereby shrink the silicone rubber insulator.
At this time, in the cold-shrink connection method, since the silicone rubber is used as the insulator for the cable accessory, the silicone rubber is required to have a tracking resistance that can withstand high voltage, to thereby have an excellent high voltage insulation. Furthermore, since the silicone rubber is applied to the cold-shrink connection method, it is important that the rubber expanded in diameter shrinks, at the time of pulling out the separator, to an original size close to the diameter before expansion, and thus to keep adhering to the conductor.
That is, the silicone rubber is required to have a tracking resistance to thereby give an excellent high voltage insulation, and to have good tension set. In addition to this, in order to achieve a good balance between both of those properties, it is also important not to deteriorate the mechanical properties necessary as the electric power components.
Until now, from the viewpoint of the material that can withstand high voltages, in order to improve the tracking resistance, there have been provided materials such as the one having a low viscosity dimethylpolysiloxane that bleeds onto the surface of the cured material, and the one having a hydroxyl group, alkoxyl group, and the like that directly bind to silicon atoms at both terminals of dimethylpolysiloxane (JP-A 7-57574, JP-A 09-324123, and JP-A 11-111087).
In addition, there are other ones in which an inorganic filler such as zinc carbonate, aluminum hydroxide or the like is added (JP-A 9-320342 and JP-A 4-209655) and, furthermore, the one in which a silicone resin is contained, as the one not containing an inorganic filler (JP-A 11-12470).
Moreover, there is further one that is aimed at improvement in the tracking resistance by using a nitrogen-containing organic compound such as a triazole-based compound in combination with a platinum catalyst or an inorganic filler (JP-A 9-316336).
Although these technologies surely improve the tracking resistance, any of the methods produce negative effects on the tension set, and thus a good balance between excellent tracking resistance and excellent low tension set cannot be achieved.
As the prior art for achieving a good balance between the excellent tracking resistance and the low tension set, there are disclosed the one in which a nitrogen-containing organic compound is added, and thus the tracking resistance test at 3.5 KV is satisfied while low tension set property is maintained, and furthermore the one in which the tracking resistance test at 4.5 KV is satisfied by the addition of a phenyl group-containing silicone oil. However, the prior art cannot satisfy severe quality requirements as the material constituting electric power components (JP-A 2004-18701).